Impedance pneumography (IP) provides a minimally intrusive mode of measurement for lung volume changes. In the IP recording a small high frequency current is passed through a pair of skin electrodes and another pair of electrodes is used to record the generated voltage that is proportional to the impedance (Z), which again is proportional to the lung volume (V). The current feeding and voltage measurement may also be combined into a single pair of electrodes.
Several studies have shown a linear relation between the thoracic impedance and the lung volume (ΔZ/ΔV) enabling an even pulmonary flow rate signal derivation, but the clinical applications of IP are still limited mainly to respiration rate and apnea detection in hospitalized patients. The emerging applications of IP in respiratory medicine use the measured signal to monitor the pulmonary flow and volume parameters related to airway obstruction during tidal breathing (TB). This places a strict requirement on the linearity between the measured thoracic impedance change (ΔZ) and the lung volume change (ΔV) throughout the complete lung volume range.
In the prior art the impedance pneumography has been applied mostly with deep inspirations, excluding the deep expirations that would reduce the lung volume below the level of normal functional residual capacity (FRC). At low lung volumes the IP signal is highly nonlinear relative to the lung volume. The resting lung volume at which TB occurs,
FRC, can change considerably due to many common factors, including supine posture, anatomical differences, mild obesity, and pathological conditions such as chronic obstructive pulmonary disease (COPD) and asthma. Thus, for the emerging IP applications that derive the pulmonary flow parameters, knowledge on the IP signal behaviour on the complete range of lung volume is important. The nonlinearity has been a factor reducing the clinical use of IP in flow measurement or in other uses more demanding than respiratory rate monitoring.
The article “An electrical impedance spirometer” by E. S. Goldensohn and L. Zablow, Department of Neurology, College on Physicians and Surgeons, Columbia University, and the Neurological
Institute, Presbyterian Hospital, New York City, 1958; discloses a sensor arrangement of two EKG-type electrodes on the wrists and two similar electrodes placed farther up on each arm.
The article “Impedance pneumograph recording across the arms” by Gershon Weltman and Dnald C. Ukkestad disclose a further study on measuring the respiratory signals across the arms, wherein all sensors are placed on the arms.